Tech Note : CODESYS Persistent Variables

CODESYS
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1

This is short note for new users to CODESYS on how to use local, retain, and persistent retain variables.

Local or Global Vars - Values that will be re-initialized when the PLC is rebooted or turned on and off. An example could be speed feedback from a VFD, or variable for the current temperature.

Retain Vars - Used for variables where only a cold reset will re-initialize these values. This might be a user configured setpoint where a maintenance tech could perform a cold reset to return a value to a factory state.

Persistent Retain Vars - Used for variables like system configuration, that should not re-initialize. The user could use a persistent variable for the number of run hours since the machine was last serviced.

2

One additional gotcha to watch out for, Retain does not survive a download and requires Persistent Vars. Wish there were a better way to handle this.

3

Then what is the meaning of download? When should it be used?

4

Download mean put a new program which don’t support online changes.

5

Hi, how do retain and persistent variables behave when power is lost?

6

Both are preserved on power loss. They behave the exact same in this regard.

7

Unless there is a specific reason not to do so, set the variables as both Retain and Persistent. Covers everything.

8

Since V3.3.0.1 PERSISTENT RETAIN and PERSISTENT are treated the same, no need to specify both.

Today I would just avoid PERSISTENT entirely, too many caveats and one corrupted write during a power outage will ruin your day. WAGO supports the Persistence Manager, a bit more initial setup, but a better experience overall.